Effects of foam and wind waves on microwave ocean emission

Meissner and Wentz (M09) [4] report global WindSat measurements of wind-induced emissivity change at 6, 10, 18, 23 and 37 GHz in wind speeds up to about 50 m/s (Fig. 1). Superimposed in the figure are the SSA/SPM simulations accounting for both foam and roughness effects. The agreement between computation and measurements is generally very good. Both measurements and simulations show a monotonic increase of emissivity change with wind speed up to 50 m/s, and the rate of change seem to slow down somewhat in high winds. Fig. 2 shows the comparison of WindSat measurements and SSA/SPM simulations with the foam and roughness components displayed separately. The foam effect increases monotonically with wind stress. For the vertical polarization in the WindSat configuration (incidence angle about 53°), foam is the dominant contributor of wind-induced emissivity change. The roughness contribution is positive at 6, 10 and 18 GHz with decreasing magnitude toward higher frequency; it becomes negative at 23 and 37 GHz. The null roughness contribution occurs between 18 and 23 GHz, and this range represents the ideal frequency band for passive microwave remote sensing with minimal surface roughness contamination. For the horizontal polarization, roughness contribution dominates in all five frequencies except for a small range of wind speeds near 50 m/s at 6 GHz. For microwave frequencies less than about 10 GHz, the wind speed sensitivity of roughness contribution is less than that of the foam contribution; for higher microwave frequencies, the two contributions have similar wind speed sensitivity.